Default sdcard unmounts itself

Hi

I have an odd issue where i can mount my sdcard & veiw the files on the pc with no probs but the moment i try to copy some thing on to the sdcard it unmounts itself, have tried this with 2 cards now & still the same thing??

have tried a hard reset too.

Any ideas please?

G1 ver 1.6
build drc92
And out of the chaos, a voice spoke:
"Smile and be happy, for it can always be worse".

And I smiled, and I was happy, and it did get worse

Xperia S CM10 142